Engineering Guidelines for the Assessment and Repair of Earthquake Damage in Residential Woodframe Buildings

During earthquakes, buildings and other improvements can be damaged directly by strong shaking or from earthquake-induced permanent displacements of the ground. For the purpose of this document, seismically-induced permanent ground deformation is defined as any earthquake-generated process that leads to deformations within a soil medium, which in turn results in permanent horizontal or vertical displacements of the ground surface. The following modes of seismically-induced permanent ground deformation have been documented in past earthquakes:
• Surface fault rupture
• Liquefaction
• Seismically-induced landsliding
• Seismic compression

When considering whether the above modes of permanent ground deformation may have affected a site, it is important to recognize the requisite conditions for their occurrence. With surface fault rupture, the requisite condition is simply proximity of the site to the ruptured fault. Ground displacements are naturally greatest at sites located directly over the ruptured fault, but significant secondary deformations can also occur away from the main break. Soil liquefaction requires the presence of ground water, soil materials considered susceptible to liquefaction (generally sands, gravels, and low plasticity silts at low densities), and dynamic loading of sufficient amplitude and duration to trigger liquefaction in those materials. The requisite conditions for landsliding are the presence of sloping ground and the presence of combined static and dynamic shear stresses that exceed material strengths. Seismic compression requires relatively strong shaking and unsaturated soil.